1. Field of the Invention
The present invention relates to a modulation controlling circuit for supplying a modulating signal to an external modulator that outputs a modulated optical signal, while setting a proper operating point in the external modulator.
2. Description of the Related Art
In recent years, light sources such as the DFB laser having sharp wavelength selectivity and capable of adjusting the oscillation wavelength freely have been put into practical use and techniques of suppressing a spectrum variation due to dispersion characteristics and nonlinear effects that are specific to optical fibers have been applied to optical transmission systems.
The wavelength division multiplexing, which makes it possible to adjust to increase in the demand for services of multimedia, B-ISDN, etc. flexibly in an inexpensive manner, has come to be applied positively to trunk line systems of such optical transmission systems.
Therefore, to prevent spreading of an oscillation spectrum due to a phenomenon that the refractive index varies with the modulating current or temperature when a light source as mentioned above is modulated directly, an LN (LiNbO3) external modulator, for example, is used in a transmitting part of a node apparatus of such a trunk line system.
FIG. 6 shows an example configuration of a transmitting part that incorporates an LN external modulator.
In FIG. 6, an exit aperture of a light source 60 is connected to an incidence aperture of an LN external modulator 62 via an optical fiber 61-1. An exit aperture of the LN external modulator 62 is connected to the immediate succeeding transmission section of an optical transmission path via an optical fiber 61-2. A data signal representing a sequence of transmission information and a clock signal that is synchronized with the transmission signal are input to first and second inputs of a modulator driving part 63, respectively. The output of the modulator driving part 63 is connected to one modulation input of the LN external modulator 62 via a bias-T circuit 64. The other modulation input of the LN external modulator 62 is grounded via a cascade connection of a resistor 65 and a capacitor 74. A monitoring terminal of the LN external modulator 62 is not only grounded via a cascade connection of a resistor 66 and a battery 75 but also connected to a first input of a phase comparator 68 via a band-pass filter 67. The output of a pilot signal generator 69 is connected to a second input of the phase comparator 68 and a third input of the modulator driving part 63. The output of the phase comparator 68 is connected to a bias terminal of the bias T-circuit 64 via a low-pass filter 70.
The LN external modulator 62 is composed of the following components:
Two optical waveguides 71-1 and 71-2 that are formed parallel between the incidence aperture and the exit aperture.
A photodiode 72 that is in weak optical coupling with the exit aperture, whose cathode is grounded, and whose anode is connected to the above-mentioned monitoring terminal.
A waveguide length varying part 73 for varying the difference between the optical waveguide lengths of the respective optical waveguides 71-1 and 71-2 in accordance with a current flowing between the above-mentioned first and second inputs.
In the transmitting part having the above configuration, the pilot signal generator 69 generates a pilot signal (for simplicity, it is assumed to be a sine wave having a frequency of 1 kHz) constantly. The modulator driving part 63 generates an NRZ signal by sampling, in synchronization with the clock signal, a bit string that is given as the above-mentioned data signal, and generates a modulating signal by amplitude-modulating the NRZ signal in accordance with the instantaneous value of the pilot signal.
On the other hand, in the LN external modulator 62, a laser beam that is emitted from the light source 60 and supplied via the optical fiber 61-1 is branched once by the optical waveguides 71-1 and 71-2, then recombined, and finally output to the immediate succeeding transmission section of the optical transmission path via the optical fiber 61-2.
The difference between the optical waveguide lengths of the respective optical waveguides 71-1 and 71-2 is given as a periodic function corresponding to the sine of the instantaneous value of the modulating signal that is supplied from the modulator driving part 63 via the bias T-circuit 64.
That is, the luminance of a laser beam that is output to the above-mentioned immediate succeeding transmission section via the optical fiber 61-2 (for simplicity, hereinafter referred to simply as xe2x80x9coutgoing beamxe2x80x9d) is amplitude-modulated so as to have opposite phases in periods when the NRZ signal included in the modulating signal has logical values xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d respectively, in a state that the operating point of the LN external modulator 62 (waveguide length varying part 73) is set properly as shown in FIG. 7A.
However, for example, in a state that the operating point of the LN external modulator 62 (waveguide length varying part 73) is not set at a proper point as shown in FIG. 7B or 7C, the luminance of an outgoing beam has phases that depend on the offset of the operating point and is given as a function of time on which the pilot signal is superimposed.
The photodiode 72 outputs a monitoring signal that represents the luminance of such an outgoing beam in the form of an instantaneous value. The band-pass filter 67 extracts the pilot signal component from the components of the monitoring signal.
The phase comparator 68 compares the phases of this pilot signal component and the pilot signal as generated by the pilot signal generator 69 and generates an error signal that indicates the difference between the two pilot signals in the forms of an instantaneous value.
The low-pass filter 70, which has a passband that is lower than the frequency of the two pilot signals, feeds back (negatively) a component of the error signal that passes through the low-pass filter 70 to the LN external modulator 62 via the bias T-circuit 64.
In other words, even if the operating environment such as the temperature varies, the operating point of the LN external modulator 62 (waveguide length varying part 73) is kept proper in a stable manner and hence the transmission quality is kept high.
In many optical transmission systems of the above conventional example, the data signal is given as the product of a PN series that is used for, for example, securing confidentiality and a bit string that represents transmission information.
Therefore, a frequency spectrum of the modulating signal has a line spectrum (see FIG. 8A) whose envelope component is given by a function E(f) of frequency f that is represented by the following Equation (1) and whose interval xcex94 on the frequency axis is represented by the following Equation (2) in a case where, for example, the number of stages of a shift register used for generation of the PN series is n and the period of the clock signal (i.e., the reciprocal of the bit rate of the transmission information) is T:
E(f)=(sin xcfx80fT/xcfx80fT)2xe2x80x83xe2x80x83(1)
xcex94=1/{(2nxe2x88x921)T}xe2x80x83xe2x80x83(2)
That is, the interval xcex94 has a smaller value when the bit rate of the transmission information (i.e., the frequency of the clock signal) is smaller.
Therefore, where the bit rate is as large as 10 Gbits/sec, for example, the probability that a component (hereinafter referred to simply as xe2x80x9cparticular componentxe2x80x9d) of the line spectrum that is closest to the pilot signal component on the frequency axis exists in the passband of the band-pas filter 67 is very low (see FIG. 8B) and the probability that a difference component between the particular component and the pilot signal component on the frequency axis exist in the passband of the low-pass filter 70 is also low.
However, where the bit rate is as low as 155 Mbits/sec, for example, it is highly probable that the particular component of a line spectrum as mentioned above exists in the passband of the band-pass filter 67 and that a difference component between the particular component and the pilot signal component exists in the passband of the low-pass filter 70 (see FIGS. 9A and 9B).
Therefore, depending on the combination of the above-mentioned bit rate and the frequency of the pilot signal (sine wave), the feedback control system formed by the band-pass filter 67, the phase comparator 68, the low-pass filter 70, and the bias T-circuit 64 between the monitoring terminal and the first input of the LN external modulator 62 may not be able to keep the operating point of the LN external modulator 62 at a proper point. Further, limitations may be imposed on the operation environment such as the temperature and desired high transmission quality may not be obtained in a stable manner.
For example, a proper operating point can be obtained in a stable manner by establishing a proper combination of the bit rate and the frequency of the pilot signal.
However, in practice, there occur cases where it is difficult to set the frequency of the pilot signal at a proper value due to the following technical limitations:
To prevent induced noise due to the use of a commercial power supply and to make the response speed of the above-described feedback control system at a value suitable for practical use, it is desirable that the frequency of the pilot signal be higher than that of the commercial power supply (50 or 60 Hz) by one order or more.
Where high-density assembling is required in surface mounting, the frequency of the pilot signal should be so high that hardware can be constructed by using small parts that enable such mounting.
To attain desired transmission quality even with turbulence of waveforms such as a minimum frequency component of the modulating signal and a sag that may accompany the waveform of the modulating signal, it is desirable that the frequency of the pilot signal be lower than several tens of kiloheltz by about one order.
As for the above-mentioned bit rate, in recent years, in particular, optical transmission systems to which the wavelength division multiplexing is applied have come to be required to flexibly adapt to a wide transmission rate range of 155.52 Mbits/sec to 9.953 Gbits/sec, for example, as in the case of xe2x80x9cbit selectxe2x80x9d and xe2x80x9cbit free.xe2x80x9d Therefore, it is difficult to change the bit rate in practice though possible technically.
For example, the following methods are available as methods for obtaining a proper operating point in a stable manner.
Setting the passband width of one or both of the band-pass filter 67 and the low-pass filter 70 as narrow as possible.
Sharpening the filtering characteristic at the boundary between the passband and the rejection band of each of the above filters and increasing the accuracy of frequency and the stability of the pilot signal and the clock signal.
However, it is difficult to implement the above methods, because those methods increase the circuit scales of the band-pass filter 67 and the low-pass filter 70 and it is necessary to use, as an oscillator to provide a frequency reference to be used in generating the pilot signal and the clock signal, an expensive high-stability crystal oscillator capable of stable operation independent from the environmental changes but large in physical size.
In summary, in the conventional example, since noise having a frequency equal to the difference between the frequencies of the pilot signal and the component of the above-mentioned line spectrum that is closest to the pilot signal on the frequency axis can be superimposed on the error signal, the extinction ratio of an optical signal output from the LN external modulator 62 lowers, which may degrade the transmission quality and disable desired wide-band transmission.
An object of the present invention is to provide a modulation controlling circuit capable of high-quality modulation in a wide bit rate range without substantially altering the basic hardware configuration and scale.
Another object of the invention is to stably keep the operating point of an external modulator at a proper operating point under a feedback control.
Another object of the invention is to promptly start an external modulator and keep its operation with stability at a proper operating point even where it requires a long time to place a filtering unit into a stationary state at its startup.
Another object of the invention is to keep the operating point of an external modulator at a proper point with stability and high accuracy.
Still another object of the invention is to keep the operating point of an external modulator at a proper operating point with stability even in a case where phase values of transmission information and a clock signal to be synchronized with the transmission information varies or the bit rate of the transmission information varies.
Another object of the invention is to simplify in configuration and reduce in scale one or both of the hardware and software of a filtering unit.
A further object of the invention is to stably keep the operating point of an external modulator at a proper operating point even where the bit rate of a sequence of transmission information is extremely low or a bit pattern given as a sequence of the transmission information may variously changes.
Another object of the invention is to keep the operating point of an external modulator at a proper operating point with stability even if an environmental condition such as a power supply voltage or the temperature varies.
Another object of the invention is to stably keep the operating point of an external modulator at a proper operating point without an increase in hardware scale or complication in hardware configuration even if a filtering unit is provided.
Another object of the invention is to heighten the response speed at the startup irrespective of filtering characteristics to be attained by a filtering unit within a stationary period.
Still another object of the invention is to heighten the response speed at the startup without complicating the hardware or software configuration.
Yet another object of the invention is to heighten the response speed without altering the basic hardware configuration in a case where a filtering unit performs filtering according to stored logic.
A further object of the invention is to improve and keep the characteristics and performance of an external modulator in an optical transmission system or a measuring instrument to which the invention is applied without losing advantages.
The invention provides a modulation controlling circuit comprising a feedback path for setting the operating point of an external modulator at a point where a phase difference between a first pilot signal superimposed on a modulating signal and a second pilot signal to be superimposed on an optical signal output from the external modulator according to the modulating signal is suppressed; and a filtering unit for suppressing components of the modulating signal that may exist in the passband of the feedback path according to turbulence of waveforms of the modulating signal.
In this modulation controlling circuit, the operating point of the external modulator can be stably kept at a proper operating point under the feedback control performed via the feedback path.
The invention provides a modulation controlling circuit comprising a unit for setting the operating point of an external modulator at a reference operating point, feeding back a phase difference as a representative of an error of the operating point to the reference operating point via a feedback path, and refraining from feeding back the phase difference until being placed into a stationary state at the startup.
In this modulation controlling circuit, the external modulator promptly starts its operation and stably operates at a proper operating point even where it requires a long time to place the filtering unit into a stationary state.
The above object is achieved by the modulation controlling circuit where a pilot signal superimposed on a modulating signal is extracted from the modulating signal to employ it as the first pilot signal.
In this modulation controlling circuit, the operating point of the external modulator is kept at a proper operating point with stability and high accuracy.
The above object is achieved by the modulation controlling circuit where at the startup the operating point of the external modulator is maintained at a reference operating point and within the stationary state an operating point in replace of the reference operating point is maintained under the feedback control.
In this modulation controlling circuit, the external modulator promptly starts its operation and stably operates at a proper operating point even in a case where it requires a long time to place a filtering unit into a stationary state.
The above objects are achieved by the modulation controlling circuit where the first pilot signal is supplied in synchronization with a sequence of transmission information to be given as a modulating signal.
In this modulation controlling circuit, it is possible to stably keep the operating point of the external modulator at a proper operating point even in a case where the phases values of the transmission information and the clock signal to be synchronized with the transmission information varies or the bit rate of the transmission information varies.
The above objects are achieved by the modulation controlling circuit where the cut-off frequency of the filtering unit is set at a value lower than the minimum frequency in the occupied band of the modulating signal and the passband of the filtering unit is set lower than the cut-off frequency.
The modulation controlling circuit realizes simplification in configuration and reduction in scale of one or both of the hardware and software of the filtering unit.
The above objects are attained by the modulation controlling circuit where the filtering unit suppresses the components of the modulating signal that may exist in the passband of the feedback path when a length of a period where a logical value of the modulating signal is kept at a constant value is at maximum.
In this modulation controlling circuit, even in a case where the bit rate of a sequence of transmission information is extremely low or a bit pattern given as a sequence of transmission information can variously changes, the operating point of the external modulator is stably kept at a proper operating point.
The above objects are achieved by the modulation controlling circuit where the filtering unit has such a wide bass band that it is possible to maintain an operating point by feedback via the feedback path at a maximum variation speed of the operating point of the external modulator.
In this modulation controlling circuit, the operating point of the external modulator is kept at a proper operating point with stability even when an environmental condition such as a power supply voltage or the temperature varies.
The above objects are achieved by the modulation controlling circuit where the filtering unit is constituted as a non-recursive digital filter.
In this modulation controlling circuit, the operating point of the external modulator is kept at a proper operating point with stability without an increase in scale or complication in configuration of the hardware.
The above objects are achieved by the modulation controlling circuit where the time constant of the feedback path is kept at a small value within a predetermined period at the startup.
The modulation controlling circuit makes it possible to heighten the response speed of at the startup irrespective of filtering characteristics to be attained by the filtering unit during a stationary period.
The above objects are attained by the modulation controlling circuit where the interval at a small value in time series to be a reference of the filtering is maintained at a small value within a predetermined period at the startup.
The modulation controlling circuit makes it possible to heighten the response speed at the startup without complicating the hardware or software configuration.
The above objects are achieved by the modulation controlling circuit where one or both of the number of operands of the filtering and the accuracy of the operands are maintained at a small value within a predetermined period at the startup.
In this modulation controlling circuit, it is possible to heighten the response speed at the startup without altering the basic hardware configuration when the aforementioned filtering is performed according to stored logic.